How can I combine the readings of multiple lidars into 1 point cloud?
A high-level solution is to use the NVIDIA DriveWorks SDK.  NVIDIA's Developer Blog has a good post, "Point Cloud Processing with NVIDIA DriveWorks SDK".

The NVIDIA DriveWorks SDK contains a collection of CUDA-based low level point cloud processing modules optimized for NVIDIA DRIVE AGX platforms. The DriveWorks Point Cloud Processing modules include common algorithms that any AV developer working with point cloud representations would need, such as accumulation and registration.

I believe you don't need the AGX hardware for the problem you posted.